Emulsions are an important product type in the field of cosmetic, dermatological and/or pharmaceutical preparations. Cosmetic preparations are essentially used for skincare. Skincare, in the cosmetics sense, refers to instances in which the natural function of the skin, as a barrier against environmental influences (for example dirt, chemicals, microorganisms) and against the loss of endogenous substances (for example water, natural fats, electrolytes), is strengthened or restored. Impairment of this function can lead to increased absorption of toxic or allergic substances or to attack by microorganisms and consequently to toxic or allergic skin reactions.
One aim of skincare is also to replenish the loss of fat and water from the skin caused by daily washing and to obtain and/or restore the softness and smoothness of the skin. This is especially important if the natural regeneration ability is inadequate. Furthermore, skincare products should protect against environmental effects, in particular against sun and wind, and delay skin aging. The sum of these skincare effects is generally summarized in cosmetics under the term “skin conditioning”.
Cosmetic preparations are also used as deodorants. Pharmaceutical topical compositions generally comprise one or more medicaments in an effective concentration. For the sake of simplicity, and for a clear distinction between cosmetic and medicinal use and corresponding products, reference is made to the legal provisions of the Federal Republic of Germany (for example Cosmetics Directive, Foods and Drugs Act).
In recent years, cosmetic wet wipes have gained increasing importance due to their exceptionally easy and convenient usability. Initially, wet wipes for cleansing purposes were almost exclusively represented in the cosmetics market; these comprised primarily aqueous, surface-active impregnation solutions. More and more, however, care products are also appearing on the market; these are based on impregnation emulsions and thus additionally comprise a caring oil component.
Most of these cosmetic wet wipes for body care and face care are impregnated with emulsions which have been prepared by the PIT emulsification method (K. Shinoda, H. Kunieda, Phase properties of emulsions: PIT and HLB, Encycl. of Emulsion Technology, 337-367 (1), 1983 or Th. Förster, F. Schambil, W. von Rybinski, J. Disp. Sci. and Technology, 13(2), 183-93 (1992)). The PIT method makes use of the fact that, in an oil-in-water (O/W) emulsion which is stabilized by nonionic emulsifiers comprising polyethylene glycol, it is possible to induce a phase inversion to a water-in-oil (W/O) emulsion by increasing the temperature (phase inversion; PIT: phase inversion temperature). Since the water-oil interfacial tension is extremely low in this phase inversion range, extremely finely divided oil-in-water emulsions can thus be obtained after cooling. For this, however, it is necessary for the individual components of the emulsions to be precisely matched to one another for each system to be emulsified. This means emulsifier mixtures and emulsifier concentration have to “be tailored” for different oil phases.
Finely divided and low-viscosity emulsions prepared in the foregoing manner have excellent long-term stability and are thus readily suitable as impregnation solutions for wet wipes. Systems of this type are described, for example, in EP-B-1 268 740 or in WO-A-00/04230.
A commercially supplied emulsion concentrate, which is prepared using PIT technology, is Emulgade® CM from Cognis (Düisseldorf) which is based on the ethoxylated emulsifiers ceteareth-20 and ceteareth-12 and comprises cetyl isononanoate as an oil component. This concentrate can be diluted with water to the desired use concentration. The dilute emulsion can then be used as an impregnation solution for wet wipes.
A disadvantage of these prior art impregnation solutions for wet wipes based on PIT emulsions is that they comprise ethoxylated emulsifiers. In the course of cosmetic formulations which are as natural as possible, an important aim of cosmetics research is to be able to dispense with emulsifiers comprising polyethylene glycol (“PEG”). For this reason, the search for PEG-free alternative solutions has increased.
It is also known that ethoxylated emulsifiers impart a rather aqueous feel to the skin, which has to be improved in sensory terms through the use of, for example, polyglycerol esters.
For example, WO-A-02/056841 describes PEG-free impregnation emulsions for cosmetic wet wipes based on polyol poly-12-hydroxystearates and alkyl glycosides. A commercially available combination of these emulsifiers for the described purpose is the product Eumulgin® VL 75 (Cognis). The use of these emulsifier mixtures leads to an improved soft feel of paper products that are impregnated therewith and also leads to improved sensory properties when using the wet wipes produced therewith.
When producing wet wipes, adequate preservation of the impregnation solutions is very important in order to prevent the build-up of germs. Here, the preservation has to be sufficient that both the impregnation solutions themselves, and ultimately also the impregnated wet wipes are protected against the build-up of germs.
Preferred preserving mixtures used for this purpose in impregnation solutions are typically mixtures of alkyl paraben esters and phenoxyethanol, as are commercially available, for example under the trade names Euxyl® K 300 (Schülke & Mayr) and Phenonip® (Clariant).
The described high requirements with regard to reliable preservation of impregnation solution and wet wipes make it necessary for relatively large amounts of these alkyl paraben ester/phenoxyethanol mixtures to generally be used in the finished impregnation solutions (0.5 to 1.0% by weight). Ideally, the total amount of preservative should be incorporated when producing emulsion concentrates. This means that it is possible to simply establish the desired use concentration of the impregnation solution through dilution with water.
If one considers that the oil phase content of impregnation solutions is typically in the range from 3 to 10% by weight, then it will be appreciated that in 50 to 80% strength concentrates, approximately 5 to 15% by weight of Euxyl® K 300 or Phenonip® have to be present in order to ensure adequate preservation.
It is known that the use of these alkyl paraben ester/phenoxyethanol mixtures has an effect which is detrimental to the emulsion since these compounds are very interface-active and compete with emulsifier molecules for a place at the oil-water interface.
This detrimental effect on the emulsion is exacerbated in the case of impregnation emulsions for wet wipes as a result of the large amounts of these preservatives required and the low viscosities of the impregnation solutions. This makes the production of stable finely divided emulsions, which comprise adequate amounts of preservatives, very difficult.
Thus, it was found, for example, that, using the emulsifier combination described in WO-A-02/056841 (e.g. Eumulgin® VL 75) and incorporating the required amounts of the specified preservatives, it was not possible to obtain impregnation solutions with adequate stability (see comparative examples).
There is thus a continued need to provide PEG-free emulsifier systems which permit the formulation of impregnation solutions for producing cosmetic wet care wipes with an adequate amount of preservative. In addition, the emulsifier systems should permit the formulation of a large number of different oils.